GONE IN A BLAZE OF GLORY: THE DEMISE OF COMET C/2015 D1 (SOHO)

Abstract

We present studies of C/2015 D1 (SOHO), the first sunskirting comet ever seen from ground stations over the past half century. The Solar and Heliospheric Observatory (SOHO) witnessed its peculiar light curve with a huge dip followed by a flareup around perihelion: the dip was likely caused by sublimation of olivines, directly evidenced by a coincident temporary disappearance of the tail. The flareup likely reflects a disintegration event, which we suggest was triggered by intense thermal stress established within the nucleus interior. Photometric data reveal an increasingly dusty coma, indicative of volatile depletion. A catastrophic mass loss rate of $\sim$10$^{5}$ kg s$^{-1}$ around perihelion was seen. Ground-based Xingming Observatory spotted the post-perihelion debris cloud. Our morphological simulations of post-perihelion images find newly released dust grains of size $a \gtrsim 10$ $\mu$m in radius, however, a temporal increase in $a_{\min}$ was also witnessed, possibly due to swift dispersions of smaller grains swept away by radiation forces without replenishment. Together with the fading profile of the light curve, a power law dust size distribution with index $\gamma = 3.2 \pm 0.1$ is derived. We detected no active remaining cometary nuclei over $\sim$0.1 km in radius in post-perihelion images acquired at Lowell Observatory. Applying radial non-gravitational parameter, $\mathcal{A}_{1} = \left(1.209 \pm 0.118 \right) \times 10^{-6}$ AU day$^{-2}$, from an isothermal water-ice sublimation model to the SOHO astrometry significantly reduces residuals and sinusoidal trends in the orbit determination. The nucleus mass $\sim$10$^{8}$--10$^{9}$ kg, and the radius $\sim$50--150 m (bulk density $\rho_{\mathrm{d}} = 0.4$ g cm$^{-3}$ assumed) before the disintegration are deduced from the photometric data; consistent results were determined from the non-gravitational effects.